Neutron Lifetimes Could Yield Insights into "Weak Force"

New precision measurements of the neutron lifetime could yield clues about how subatomic particles coalesced into the elements that formed our universe after the so-called Big Bang, according to M. Scott Dewey, a physicist at the National Institute of Standards and Technology who spoke at the Washington APS meeting. Neutrons are stable in an atomic nucleus, but when they are unbound, as they were at the birth of the universe, they decay. The rate at which these early neutrons decayed determined, in part, the ratio of abundances of elements in the universe.

The current measured neutron lifetime is 14 minutes, 47.0 seconds, with an error margin of 2.0 seconds. In collaboration with scientists at France's Institute Laue Langevin and Russia's Petersburg Nuclear Physics Institute, Dewey and colleagues are reducing the uncertainty of neutron lifetime measurements. NIST also expects to make a new measurement within the next few months.

Among the benefits of this research is a clearer view of the "weak force", one of four forces in nature. The weak force is responsible for beta decay radioactivity. For instance, better measurements could help resolve the question surrounding the phenomenon of parity violation. However, many physicists believe that during the first instants of the Big Bang, there was equal probability of the weak force being right-handed or left-handed.

According to Dewey, recent experiments seem to support the Standard Model and the notion that the weak force was always left-handed. However, more sensitive measurements using neutrons and other radioactive decays will be required to provide further insight into the question. Since neutrons have no charge, they are somewhat difficult to manipulate. Future experiments to be carried out at the NIST National Cold Neutron Research Facility could improve the accuracy of neutron lifetime measurements by another factor of 10 or more, thanks to specially designed instruments at that facility.

Neutrons are liberated from uranium atoms inside a 20-megawatt research reactor at NIST. These are chilled and slowed by passing through a hydrogenous "moderator" cooled to a very low temperature. Glass tubes with nickel-coated interiors guide the neutrons to various experiment stations in the facility.

Dewey and his colleagues rely on a magnetic trap at the end of a long neutron guide coming from the reactor. If a neutron happens to decay in the trap's cylinder, the resulting proton is contained within the trap's magnetic field. An electric field around the trap then deflects the positively charged protons into a detector. The vast majority of neutrons do not decay, but slam straight through the trap into another detector used to monitor the total flow of neutrons. The NIST team is presently eliminating possible sources of errors in their latest experimental data.

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